Fumigants are widely used for the disinfestation, and protection against infestation, that is usually required to protect particulate materials (such as grain) and other stored produce (including durable and perishable foodstuff), porous bulk materials (for example, soil or timber) and spaces (typically, empty buildings). An ideal fumigant should be toxic to insects, mites, nematodes, bacteria, fungi and moulds. It should be effective in low concentrations. It should have a low absorbtion by materials in the fumigated region. It should have a low mammalian toxicity and leave either no residue or an inert residue. In addition, the ideal fumigant should present no difficulties as far as safe handling is concerned, and it should not adversely affect the commodity or space that is being fumigated.
No fumigant meets all of these "ideal" criteria. The two fumigants most commonly used in the fumigation of grain, other particulate materials, fruit and timber are phosphine and methyl bromide. Phosphine is the preferred fumigant for grain stores and the like because it is effective against grain pests and leaves little residue (which is essentially a harmless phosphate). However, phosphine is spontaneously combustible when its concentration exceeds a relatively low value.
Methyl bromide is more toxic to grain pests than phosphine when used for short periods of fumigation, but phosphine is more toxic to grain pests when long term fumigation is effected. Methyl bromide has a lower flammability than phosphine, but recent work has shown that methyl bromide depletes the ozone layer. Thus approval of methyl bromide as a fumigant is currently under review, following the Montreal protocol.
Other fumigants that have been used against grain pests include acrylonitrile, carbon disulphide, carbon tetrachloride, chloropicrin, ethylene dibromide, ethylene dichloride, ethylene oxide, hydrogen cyanide and sulphuryl fluoride. It will be noted that a halogen is present in the majority of these "conventional" fumigants, none of which has the "ideal" fumigant properties.
For many years, there has been a. constant seeking of new fumigants and there is no doubt that the quest for improved fumigants will continue.